An LED (Light Emitting Diode) is widely used as a device having low power consumption, contributing to reduction of carbon dioxide, and exhibiting high durability, namely as a device that contributes to environmental protection and energy saving. A package incorporating such an LED chip is mounted onto a wiring substrate (module substrate) and is used for not only backlights in electronic equipment, e.g., a large-sized display, a cellular phone, a digital video camera, and a PDA, but also for road illumination, general illumination, and so on. In trying to increase the emission intensity of the LED device, heat generates in a larger amount, and the emission efficiency of the LED device reduces with heating. Therefore, the LED device is required to include a structure capable of effectively dissipating heat.
A current LED luminescent device employs a ceramic substrate, a silicon substrate, or a metal substrate as an LED package substrate. However, known structures employing the ceramic substrate or the silicon substrate have the problems that heat is not effectively dissipated because ceramics and silicon have lower thermal conductivity than metals, such as copper, that those substrates are relatively expensive, and that a difficulty occurs in processing of the substrates.
FIG. 25 is a side sectional view illustrating a lighting fixture disclosed in Patent Document 1. An illustrated metal-core printed circuit board includes a metal core and a printed circuit that is formed, by processing a copper foil into a circuit pattern, on the metal core with an insulating layer interposed between them. The insulating layer is made of a heat-resistant and thermoplastic resin having a thickness of about 100 μm, the resin being any of polyether ether ketone, polyether imide, and polyether sulfone. A light emitting diode is placed on and fixed to a bottom surface of a recess of the metal-core printed circuit board, and terminals of the light emitting diode are connected to the printed circuit. A transparent acrylic resin is filled in the recess. Thus, it is known to use the heat-resistant and thermoplastic resin as the insulating layer, but the insulating layer made of the resin has a problem with heat dissipation.
Hitherto, a liquid material obtained by adding a white inorganic pigment to a thermo-setting resin made of an organic material is proposed as a composition capable of forming a high-reflectance solder resist film (e.g., Patent Document 2). The white inorganic pigment has a particle diameter of 0.3 μm or smaller, for example. The liquid material containing the pigment having such a particle diameter cannot be coated with an ink jet applicator, a dispenser, or a spray coater, and screen printing has to be employed. Moreover, heat dissipation performance of organic materials is generally about 0.3 w/m·k and is inferior to that of inorganic materials (e.g., silicon dioxide (SiO2): about 1.5 w/m·k, titanium dioxide: about 8 w/m·k, and zinc oxide: about 50 w/m·k). In addition, a wiring substrate including an insulating layer made of an organic material has problems with heat resistance and deterioration due to ultraviolet rays or long-term use (i.e., durability).
Patent Document 3 discloses an LED package employing, as a base material, a Cu substrate including a thin-film Cu wiring layer, which is integrally formed on its surface, while an insulating layer made of silicon dioxide (SiO2) is interposed between them as a bonding material. FIG. 24 illustrates an LED package disclosed in Patent Document 3. FIG. 24(A) is a side sectional view of a central portion of the package, and FIG. 24(B) is a plan view of the central portion. In the Cu substrate mounting the LED chip thereon, a concave recess receiving the mounted LED chip is formed by pressing. However, SiO2 is hard and tends to fracture during the pressing. Hence there is a problem that SiO2 is unsuitable for the pressing. Moreover, an Ag plating is coated on the surface of the Cu wiring layer to give the surface with light reflectance, thus resulting in a higher cost.
Recently, a heat pipe for cooling a heat source through evaporation and condensation of an enclosed coolant has been proposed for mounting into even a small space. Patent Document 4 proposes a heat pipe comprising a flat-plate main unit including a vapor diffusion channel through which the evaporated coolant is diffused and a capillary channel through which the condensed coolant is returned, a temperature measuring unit that measures a temperature difference between at least two locations of the main unit, a comparison unit that compares the temperature difference with a predetermined threshold and that outputs a comparison result, and a determination unit that determines, in accordance with the comparison result, an operating state of the main unit on the basis of a cooling capability of the heat pipe, wherein the vapor diffusion channel diffuses the evaporated coolant in a horizontal direction, and the capillary channel returns the condensed coolant in a vertical direction or in the vertical and horizontal directions.